According to Hirano's theory, what primarily determines the mode of vocal fold vibration?

Hirano's theory, particularly with respect to the cover-body theory of the vocal folds, highlights the importance of the interplay between the different mechanical properties of the vocal fold layers. According to this theory, the vocal folds consist of multiple layers, each contributing unique characteristics to their overall function. The five layers of the vocal fold structure include the epithelium, the lamina propria with its superficial, intermediate, and deep layers, and the thyroarytenoid muscle.

The mode of vocal fold vibration is determined by how these layers interact, which includes their elasticity, viscosity, and tension. This interaction allows for the modulation of pitch and quality of voice based on varying vocal fold configurations during phonation. Thus, it's the complex combination of these mechanical properties, rather than any single factor such as tension, mass, or length, that primarily influences how the vocal folds vibrate. Understanding this complexity is crucial for comprehending how voice production works in a nuanced manner.